Will accounts be auto-upgraded or downgraded based on lead limits as well?
They can be. For the same reasons, there is a daily limit on the number of leads brought over from Facebook. For most users, that lead limit will be quite high.
However, those with very large numbers of leads per day can exceed plan thresholds as well. If that is the case, we will auto-upgrade or downgrade based on a moving 10 day average.
For the official list of plan limits, please see out End User License Agreement (EULA).